No evidence for associations between brood size, gut microbiome diversity and survival in great tit (Parus major) nestlings

Background: The gut microbiome forms at an early stage, yet data on the environmental factors influencing the development of wild avian microbiomes is limited. As the gut microbiome is a vital part of organismal health, it is important to understand how it may connect to host performance. The early studies with wild gut microbiome have shown that the rearing environment may be of importance in gut microbiome formation, yet the results vary across taxa, and the effects of specific environmental factors have not been characterized. Here, wild great tit (Parus major) broods were manipulated to either reduce or enlarge the original brood soon after hatching. We investigated if brood size was associated with nestling bacterial gut microbiome, and whether gut microbiome diversity predicted survival. Fecal samples were collected at mid-nestling stage and sequenced with the 16S rRNA gene amplicon sequencing, and nestling growth and survival were measured.Results: Gut microbiome diversity showed high variation between individuals, but this variation was not significantly explained by brood size or body mass. Additionally, we did not find a significant effect of brood size on body mass or gut microbiome composition. We also demonstrated that early handling had no impact on nestling performance or gut microbiome. Furthermore, we found no significant association between gut microbiome diversity and short-term (survival to fledging) or mid-term (apparent juvenile) survival.Conclusions: We found no clear association between early-life environment, offspring condition and gut microbiome. This suggests that brood size is not a significantly contributing factor to great tit nestling condition, and that other environmental and genetic factors may be more strongly linked to offspring condition and gut microbiome. Future studies should expand into other early-life environmental factors e.g., diet composition and quality, and parental influences.</p

The effects of prenatal corticosterone and thyroid hormone on great tit (Parus major) DNA methylation and gene expression

Offspring phenotype is determined by genotype and environmental factors such as maternal hormones during embryonic development. Avian eggs contain multiple hormones of maternal origin. Differences in egg hormone concentrations are hypothesized to cause variation in offspring fitness and survival via epigenetic mechanisms. In this context, my MSc project tested the effects of prenatal glucocorticoid ‘stress’ and thyroid hormones on offspring DNA methylation and gene expression of the glucocorticoid receptor (GCR)- and the thyroid hormone receptor (THR) -genes in a wild avian population. Great tit (Parus major) eggs were injected with corticosterone (main avian glucocorticoid) and/or thyroid hormones to simulate variation in maternal hormone deposition. Methylation status and gene expression were analyzed from longitudinal blood samples taken 7 and 14 days after hatching, as well as the following autumn. DNA methylation was analyzed by bisulfite conversion and pyrosequencing, while gene expression was assessed with RT-qPCR. Hormonal treatment did not significantly impact methylation status at the putative promoter region of the genes of interest, nor the expression of GCR and THR genes. Although higher methylation levels are known to inhibit gene expression, GCR promoter methylation correlated positively with GCR gene expression. This contrasts the canonical view of the suppressive role of promoter methylation. GCR expression correlated negatively with reproductive condition and growth. In turn THR did not exhibit any significant relationship with the examined covariates, leading to the hypothesis that TH signaling pathways might be more robust due to the crucial role of thyroid hormones in development

Early-life environmental effects on mitochondrial aerobic metabolism: an experimental brood size manipulation in wild great tits

Abstract Parental care (including postnatal provisioning) is a major component of the offspring’s early-life environment. In avian species, the number of chicks in the nest and subsequent sibling competition for food are known to affect chick’s growth, leading in some cases to long-lasting effects for the offspring. Because of its central role in converting energy, variation in the offspring’s mitochondrial metabolism could be an important pathway underlying variation in growth patterns. Here, we performed a brood size manipulation in great tits ( Parus major ) to unravel its impact on offspring’s mitochondrial metabolism and reactive oxygen species (ROS) production in red blood cells. We investigated the effects of brood size on chicks’ growth and survival, and tested for long-lasting effects on juvenile mitochondrial metabolism and phenotype. As expected, chicks raised in reduced broods had a higher body mass compared to enlarged and control groups. However, mitochondrial metabolism and ROS production were not significantly affected by the treatment either at chick or juvenile stages. Chicks in very small broods were smaller in size and had higher mitochondrial metabolic rates. The nest of rearing has a significant effect on nestling mitochondrial metabolism, yet variation in mitochondrial metabolism at the early-life stages are not associated with survival chances. The contribution of the rearing environment in determining offspring mitochondrial metabolism emphasizes the plasticity of mitochondrial metabolism in changing environments. Further studies would be needed to closely investigate what are the major environmental cues affecting the offspring mitochondrial metabolism during the growth period

No evidence for associations between brood size, gut microbiome diversity and survival in great tit (Parus major) nestlings

Background: The gut microbiome forms at an early stage, yet data on the environmental factors infuencing the development of wild avian microbiomes is limited. As the gut microbiome is a vital part of organismal health, it is important to understand how it may connect to host performance. The early studies with wild gut microbiome have shown that the rearing environment may be of importance in gut microbiome formation, yet the results vary across taxa, and the efects of specifc environmental factors have not been characterized. Here, wild great tit (Parus major) broods were manipulated to either reduce or enlarge the original brood soon after hatching. We investigated if brood size was associated with nestling bacterial gut microbiome, and whether gut microbiome diversity predicted survival. Fecal samples were collected at mid-nestling stage and sequenced with the 16S rRNA gene amplicon sequencing, and nestling growth and survival were measured. Results: Gut microbiome diversity showed high variation between individuals, but this variation was not signifcantly explained by brood size or body mass. Additionally, we did not fnd a signifcant efect of brood size on body mass or gut microbiome composition. We also demonstrated that early handling had no impact on nestling performance or gut microbiome. Furthermore, we found no signifcant association between gut microbiome diversity and short-term (survival to fedging) or mid-term (apparent juvenile) survival. Conclusions: We found no clear association between early-life environment, ofspring condition and gut microbiome. This suggests that brood size is not a signifcantly contributing factor to great tit nestling condition, and that other environmental and genetic factors may be more strongly linked to ofspring condition and gut microbiome. Future studies should expand into other early-life environmental factors e.g., diet composition and quality, and parental infuences.peerReviewe

Early-life environmental effects on mitochondrial aerobic metabolism: a brood size manipulation in wild great tits

International audienceIn avian species, the number of chicks in the nest and subsequent sibling competition for food are major components of the offspring's early-life environment. A large brood size is known to affect chick's growth, leading in some cases to long-lasting effects for the offspring, such as a decrease in size at fledgling and in survival after fledging. An important pathway underlying different growth patterns could be the variation in offspring mitochondrial metabolism through its central role in converting energy. Here, we performed a brood size manipulation in great tits (Parus major) to unravel its impact on offspring's mitochondrial metabolism and reactive oxygen species (ROS) production in red blood cells. We investigated the effects of brood size on chicks’ growth and survival, and tested for long-lasting effects on juvenile mitochondrial metabolism and phenotype. As expected, chicks raised in reduced broods had a higher body mass compared to enlarged and control groups. However, mitochondrial metabolism and ROS production were not significantly affected by the treatment either at chick or juvenile stages. Interestingly, chicks raised in very small broods were smaller in size and had higher mitochondrial metabolic rates. The nest of rearing had a significant effect on nestling mitochondrial metabolism. The contribution of the rearing environment in determining offspring mitochondrial metabolism emphasizes the plasticity of mitochondrial metabolism in regards to the nest environment. This study opens new avenues regarding the implication of postnatal environmental conditions in shaping the offspring's early-life mitochondrial metabolism

From maternal glucocorticoid and thyroid hormones to epigenetic regulation of offspring gene expression : An experimental study in a wild bird species

Offspring phenotype at birth is determined by its genotype and the prenatal environment including exposure to maternal hormones. Variation in both maternal glucocorticoids and thyroid hormones can affect offspring phenotype, but the underlying molecular mechanisms, especially those contributing to long-lasting effects, remain unclear. Epigenetic changes (such as DNA methylation) have been postulated as mediators of long-lasting effects of early-life environment. In this study, we determined the effects of elevated prenatal glucocorticoid and thyroid hormones on handling stress response (breath rate) as well as DNA methylation and gene expression of glucocorticoid receptor (GR) and thyroid hormone receptor (THR) in great tits (Parus major). Eggs were injected before incubation onset with corticosterone (the main avian glucocorticoid) and/or thyroid hormones (thyroxine and triiodothyronine) to simulate variation in maternal hormone deposition. Breath rate during handling and gene expression of GR and THR were evaluated 14 days after hatching. Methylation status of GR and THR genes was analyzed from the longitudinal blood cells sampled 7 and 14 days after hatching, as well as the following autumn. Elevated prenatal corticosterone level significantly increased the breath rate during handling, indicating an enhanced metabolic stress response. Prenatal corticosterone manipulation had CpG-site-specific effects on DNA methylation at the GR putative promoter region, while it did not significantly affect GR gene expression. GR expression was negatively associated with earlier hatching date and chick size. THR methylation or expression did not exhibit any significant relationship with the hormonal treatments or the examined covariates, suggesting that TH signaling may be more robust due to its crucial role in development. This study provides some support to the hypothesis suggesting that maternal corticosterone may influence offspring metabolic stress response via epigenetic alterations, yet their possible adaptive role in optimizing offspring phenotype to the prevailing conditions, context-dependency, and the underlying molecular interplay needs further research.peerReviewe